This invention relates to an improved design of hydraulic pumps and motors, either as a stand-alone pump or motor or in combination as a transmission, and a method of manufacturing such pumps, motors and transmissions. The inventions described herein may also be adapted for use in an integrated hydrostatic transmission (IHT), wherein a hydraulic transmission is incorporated in a single housing with output gearing and one or more axles. Hydrostatic units such as these are used in a variety of vehicle and industrial applications, including lawn and garden tractors, zero-turn units, snow throwers, and other apparatus where hydraulic and hydrostatic power may be used. Applications of the invention will be described with respect to an HST to convey the fundamental nature of the invention. 
In general an HST has a hydraulic pump and a hydraulic motor mounted in a housing. The pump and motor are hydraulically linked through a generally closed circuit; both consist of a rotatable body with pistons mounted therein. The pump and motor are rotatably mounted on a center section, which acts to form a closed circuit in conjunction with the pump and motor.
Hydraulic fluid such as oil is maintained in the aforementioned closed circuit. The HST generally has a sump or reservoir with which the closed circuit can exchange oil. This sump may be formed by the housing itself.
The pump is driven by means of a pulley and belt, a shaft, or other connective means associated with a motive source. The motive force is often provided by an internal combustion engine, but may also be an electric motor. The pump pistons engage a moveable swash plate as the pump is rotated by an input shaft driven by the external engine. Other HST designs may use other pump configurations, but the general operations are similar in that the wear surfaces associated with hydraulic and hydrostatic pumps and motors are similar to those described herein and may benefit from the teachings described herein.
In an HST, movement of the pump pistons creates movement of the hydraulic fluid from the pump to the motor, causing rotation thereof. The motor pistons are engaged against a plate that may be at a fixed angle, and rotation of the motor drives an output shaft engaged thereto. This output shaft may be linked to mechanical gearing and to one or more output axles, which may be internal to the HST housing, or may be external thereto.
The pump/motor system is fully reversible in a standard HST. As the swash plate against which the pump pistons move is moved, the rotational direction of the motor may be changed. In addition, there is a “neutral” position where the pump pistons are not moved in  an axial direction, so that rotation of the pump does not create any movement of the hydraulic fluid.
The hydrostatic closed circuit has two sides, namely a high pressure side in which oil is being pumped from the pump to the motor, and a low pressure or vacuum side, in which oil is being returned from the motor to the pump. When the swash plate angle is reversed, the flow out of the pump reverses so that the high pressure side of the circuit becomes the vacuum side and vice versa. This hydraulic circuit can be formed as porting within the housing, or internal to a center section on which the pump and motor are rotatably mounted, or in other ways known in the art. Check valves are often used as an inlet to draw hydraulic fluid into the low pressure side to make up for fluid lost, for example, due to leakage. Such check valves may be located so that they directly contact the porting or may be located separate from the porting and connected through additional bores to the closed circuit.
A center section for use in an HST or IHT may be various shapes, including a flat plate such as is shown in U.S. Pat. No. 5,392,670 or an offset design, as shown in U.S. Pat. No. 5,314,387, both incorporated herein by reference along with U.S. Pat. No. 6,122,996. It is currently preferred to manufacture such center sections from aluminum, which provides for a unit that is easy to cast and to machine to final form. Aluminum is also lighter and easier to handle than other materials, such as cast iron, which are also used to make such center sections.
This invention also relates to cast iron “end caps” which are used in stand-alone hydraulic pumps and in hydraulic transmissions, and on which the rotating cylinder block  is mounted. The end cap functions much as a center section in that it provides both porting and support for the cylinder blocks.
A problem with the aforementioned cast iron end caps is their tendency to corrode, amplified by typical operating environments. Such corrosion typically does not affect performance, but does affect visual appearance and thus customer satisfaction and ultimately unit sales.
Painting of end caps is routinely done for appearance, as the cast iron rusts readily in exposure to humidity. Painting, however, must be kept from the running surfaces to prevent undesirable contamination and degradation of performance characteristics. The gerotor and block running surfaces are typically masked, or treated, to avoid paint adhering to the surfaces; such masking and treatment requires specific facilities for painting and adds labor costs to control where the paint is applied. Some manufacturers paint the end cap after it has been assembled into a hydrostatic unit. Such painting and handling entails a variety of problems, including that of limiting paint to only the part at issue, namely the end cap, and painting of bulky and heavy hydrostatic units. Thus there is a need to eliminate the foregoing problems associated with end caps and other problems as well.
The use of aluminum for a center section or end cap has certain drawbacks. For example, aluminum is significantly less robust than cast iron. Gouging or scarring of the center section can occur more readily with aluminum under certain loading or contamination conditions. As is generally known, the center section and the rotating pump and motor may be mounted in a sump which contains the hydraulic oil used for the hydrostatic transmission. In many designs this hydraulic oil is also used to lubricate the gearing and/or differential for the transaxle. The motion of the pump and motor requires a thin hydrodynamic film of oil  where the pump and motor contact the aluminum center section. Particles in the oil can break down the hydrodynamic film and damage the center section. The problem is greater at higher temperatures where the oil viscosity is lower and thus the hydrodynamic film thickness decreases.
One alternative design that capitalizes on the advantages of an aluminum center section is to include a valve plate between the pump and the center section against which the pump runs. The valve plate would be made of a more durable material than aluminum to address the problems listed above. Valve plates used with previous hydrostatic units are generally of a bi-metal configuration, having one side composed of bronze and the other side of steel. The bronze forms a better running surface than steel. The steel substrate is required to prevent the flexure of the plate under some operating conditions. One problem that has arisen with such bi-metal valve plate construction is that the different metals have different expansion coefficients, and at elevated temperatures the relative expansion rates of the two metals cause the valve plate to bend or warp. Such bending ultimately causes performance problems, and causes such valve plates to have an operational temperature limit. This invention overcomes the limitation of the bi-metal valve plate design while retaining wear characteristics.
Additionally, the valve plate and the surface on which it is mounted must be held sufficiently flat to prevent high pressure fluid from building up under, and lifting, the valve plate. Once the valve plate is lifted, system pressure is lost, performance is reduced and the various hydraulic components may experience permanent damage. 
Therefore, it is clear that there exists a need for a method of improving the wear characteristics of the materials used in conjunction with center sections, end caps, valve plates, and other components on which hydraulic components operate.